1. Field of the Invention
This invention relates to a process and device for the continuous extraction and analysis of chemical substances from fluid using membranes directly interfaced to an analytical device and extraction of semi-volatile, non-volatile and polar substances from fluid using pressurized dense gas.
2. Description of the Prior Art
The removal of substances from fluid such as water is important for a number of reasons. Of particular interest are organic substances, since many are known to be toxic or carcinogenic or may also contribute to undesirable properties, such as poor odour or taste.
Such substances may be removed from water in order to identify and quantitate the substances to protect the public. Secondly, they may be monitored and possibly removed from water prior to the water being used for some manufacturing processes, for example, for some pharmaceutical and for some beverage production. Thirdly, it is important to remove organic substances from water for water remediation.
In general, the organic substances of interest in water are divided into three classes; volatile, semi-volatile and non-volatile. Polar substances in these three groups are the most difficult to deal with because of their affinity to water.
Organic substances, present as pollutants of interest in many water samples, are typically found in part per million and part per billion levels. Presently, in the analysis of organic substances from water, there are at least two steps. First is the extraction of the substances from the water and second is the analysis of the extract. Methods of analysis such as gas chromatography and gas chromatography-mass spectrometry are very rapid, sensitive and selective. Current extraction methods, on the other hand, are tedious, slow and often involve the use of considerable amounts of organic solvent. Loss of sample, reducing the accuracy of the method, is common.
Direct injection of the matrix containing target analytes eliminates the extraction step and the use of organic solvent, but has poor sensitivity and suffers from many interferences.
Extraction techniques include static headspace sampling, purge and trap, solid phase extraction, liquid/liquid extraction and distillation. Static headspace is not a sensitive technique and is not suitable for non-volatile substances. Purge and trap is widely used for volatiles, however, it suffers from many problems due to water vapor carry-over. Since cryogenic cooling is usually required to concentrate the analytes, freezing of the water is a problem. Water also causes interference with the chromatographic analysis and reduces column lifetimes. Foaming of the sample during purging in purge-trap methods can also be a problem. Automated equipment for multiple samples is expensive, complicated and prone to problems such as leaks. Solid phase extraction is efficient but still requires the use of organic solvent. Solid phase extraction cartridges are typically used only once for one sample and are then disposed. This generates solid waste. Liquid-liquid extraction is widely used but requires the separation of the phases following extraction. This can result in loss of sample and is labour intensive. The most serious problems with liquid-liquid extraction, are due to the large amounts of organic solvents required for extraction. Solvent costs are high and solvent use presents health risks as well as being an environmental hazard. Disposal costs for waste solvent are high. There is a great desire to reduce solvent useage in laboratories, however this has been difficult without acceptable alternative methods being available. Distillation is not an energy efficient method of extraction and is labour intensive. Loss of volatile substances often occurs with distillation.
In each of these extraction techniques, the method of analysis must still be performed as a separate operation. This is inefficient and introduces many types of errors into the analytical result. Continuous analysis of volatile, semi-volatile and non-volatile organic substances in water is not possible using batch extraction and analysis techniques.
Considerable effort has been expended developing methods for the analysis of volatile, semi-volatile and non-volatile organic pollutants in water. For example, in the United States, the Environmental Protection Agency (EPA) Priority Pollutants designated in Method of Organic Chemical Analysis of Municipal and Industrial Wastewater EPA-600/4-83-057 ( US EPA, Cinninnati, Ohio ) are analyzed by methods described in the Federal Register, Testing Methods for Evaluating Solid Waste, SW-846, 2nd Edition, US EPA Office of Solid Waste and Emergency Response. Industries in the United States are required to use these methods both in evaluating their wastewater and in applying for National Pollution Discharge Permits. Other countries, for example, Canada, Germany and Japan have similar established methods or are in the process of developing comparable methods for these same requirements. Organic pollutants normally are present in parts per billion or lower concentrations in wastewater. Using EPA methods, to achieve required sensitivities, analytes must be concentrated prior to analysis. All EPA wastewater methods are batch methods, involving separate sampling procedures, preconcentration and possibly cleanup, and finally, analysis, which is usually by gas chromatography or gas chromatography-mass spectrometry. Sample preconcentration is by purge and trap or solvent extraction. For volatile compounds, for example EPA Methods 601, 602 and 603 which are purge and trap, loss of volatile substances between sampling and analysis can occur. All other methods, for semi-volatile and non-volatile compounds specify liquid-liquid extraction using organic solvents. For example, Method 625, widely used for non-volatile pollutants, requires a preconcentration step using 300 ml methylene chloride per litre of water sample followed by evaporation to 2 ml. There is considerable concern about the amount of organic solvent required for semi-volatile and non-volatile extractions. Due to the expense involved in these methods and the batch sampling required, continuous analysis of wastewater streams for monitoring of low level priority pollutants is not practical. Interpretation of results and conclusions regarding the nature and sources of priority pollutants is therefore difficult. Separate sampling and analysis processes add greatly to the expense. Necessarily, the sample taken for analysis is a discrete sample, representing the condition of the wastewater at only one point in time. Any useful information about changes in the wastewater discharge must therefore involve expensive repetitive sampling and analysis. Afternatively, the sample may be an averaged sample, taken using specialized and expensive time-based sampling devices. However, this does not provide any information about pollutant flux.
Computers may be used to collect and analyze data on EPA priority pollutant sources, however, due to the lack of availability of an inexpensive method to generate continuous analysis data from these sources, it is impossible or very expensive to attempt to fully characterize pollutant sources, especially with respect to rapid changes in pollutant levels.
The use of semipermeable structures such as membranes can provide efficient means of extraction of organic substances from a fluid. Membranes are available in a variety of forms and shapes. Flat sheets are often used, especially for dialysis, however, the hollow fibre is a more useful geometry. A larger surface area per volume is obtained and hence more efficient extraction is possible. There are two geometries used with the hollow fibre membrane. One is referred to as the flow-over configuration. In this configuration, the stripping media flows through the fibre while the feed solution is pumped around the exterior of the fibre. The second geometry is the flow-through configuration. In this type of set-up, the stripping media flows around the exterior of the fibre while the feed solution is pumped through the fibre. The flow-through configuration results in higher linear velocities and improved surface area per volume ratio.
Hollow fibre membrane has been used for analytical and for process applications. In analytical applications, hollow fibre membranes have been studied as a method of direct sample introduction into a mass spectrometer. This system is being studied in analytical chemistry as well as in biotechnology and microbiology. Membrane introduction mass spectrometry has allowed continuous monitoring of chemical or biochemical reactions. However, in this situation, vacuum is used to strip the analytes from the membrane and there is no means of concentrating the analytes. Sensitivity is therefore limited by this technique.
Gas chromatography is a more prevalent and less expensive technique than direct mass spectrometry. However, in all prior methods using membrane, the continuous extraction and continuous analysis of organic substances in aqueous fluid using gas chromatography has not been possible.
Multiplex gas chromatographic analysis has been used to permit continuous monitoring of gaseous components with rapidly changing concentrations . It has also proven very useful for trace analysis and eliminates the need for a preconcentration step. Multiplex gas chromatographic analysis of organic substances in water would permit continuous measurement with enhanced sensitivity. This has not been possible, however, because of the interference effect of the water on measurement.
Pressurized dense gases such as supercritical fluid have been shown to effectively extract non-volatile, semi-volatile and polar substances from liquid and solid matrices. This is due to the strong solvating properties of the supercritical fluid. Carbon dioxide can be used in this application because of it's low critical temperature and pressure. It is commonly selected since it is inexpensive, easily available and non-toxic. However, the use of pressurized dense gas such as supercritical carbon dioxide to remove substances from aqueous samples is very difficult and supercritical fluid extraction methods developed-for extraction of organics from water are very limited. Water dissolves in the carbon dioxide and this causes restrictor freezing and plugging. An indirect supercritical fluid extraction method has been developed using a solid sorbent to transfer analytes from water to the supercritical fluid. However, this method includes three time consuming processes: adsorption, drying, and extraction. Loss of analytes can occur during the drying procedure.
U.S. Pat. No. 4,250,331 (1981) to Shimshick shows that dense gas can remove non-volatile polar substances from aqueous fluid by direct contact between the two phases. However, Shimshick only considered the batch process. In the batch process, the extraction is very slow because of poor contact between the phases. Phase separation is difficult. Batch extraction of larger volumes of sample involves large and expensive high pressure extraction vessels. Large vessels used under supercritical fluid pressure and temperature conditions must be used with extreme caution for safety reasons. In the batch process, continuous extraction or analysis of substances in a stream of the aqueous fluid is not possible.
Dense gases under pressure, such as supercritical fluids are typically produced with pressures in the range from 1,500 psi to 10,000 psi. The use of supercritical fluids therefore has not been considered compatible with membrane, due to the limited ability of semipermeable membrane to withstand such pressures.